1. Field of the Invention
The present invention relates to thermally labile surfactants prepared by Diels-Alder (hereinafter “DA”) reactions. In particular, the invention relates to the sodium sulfonate thermally cleavable surfactant and processes for their preparation.
2. Description of the Related Art
Over the past decade, the development of cleavable surfactants has been a growing field in surfactant science. As the name implies, cleavable surfactants are molecules that undergo a chemical or physical change of the parent molecular structure resulting in a change and/or loss of surface-active behavior. Hence, the production of commercially available cleavable surfactants may find utility in industrial practices where foaming or persistent surface-active properties must be diminished after their initial use in “green” chemistry where degradability is of primary concern, and in biomedical drug delivery where surfactants could be removed through biological mechanisms.
To overcome these shortcomings, we describe here the synthesis and characterization of two new surfactant compositions which incorporate a thermally cleavable DA adduct as the chemical weak-link within the surfactant molecular structure. In particular, we have utilized the reversible DA reaction between functionalized furans and maleimides as the basis for thermally cleavable materials. We have previously reported the integration of furan-maleimide DA adducts into molecules to produce thermally responsive encapsulating polymers, foams, and adhesives as well as dendrons and dendrimers which reversibly self-assemble (see U.S. Pat. Ser. Nos. 6,271,335; 6,337,384; and 6,403,753; U.S. published application Ser. No. 20030116272; and Aubert, J. H., Journal of Adhesion, 2003, v. 79, pp. 609-616; McElhanon, J. R., et al., Journal of Applied Polymer Science, 2002, v. 85, p. 1496; and McElhanon, J. R., et al., Organic Letters, 2001, v. 3(17), p. 2681). Also, similar thermally reversible DA adducts have been reported incorporated into other responsive polymers (see Chen, X., et al., Macromolecules, 2003, v. 36, p1802-1807; and Chen, X., et al., Science, 2002, v. 295, p1698-1702).
We have previously reported on (McElhanon, J. R., et al., Langmuir, 2005, v. 21, pp. 3259-3266) the synthesis and characterization of surfactants that possess a furan-maleimide DA thermally activated weak-link. These surfactants possessed either carboxylate or phenolate polar head groups and a 12 carbon linear alkyl chain that served as the hydrophobic tail section. These surfactants all require deprotonation of the head group through addition of an excess of potassium hydroxide to achieve solubility in water. We present here the synthesis and characterization of two sulfonate-based sodium salt DA surfactants that require no deprotonation step or addition of hydroxide and are soluble in water and/ or polar organic solvents. The surfactants degrade into hydrophilic and hydrophobic fragments after exposure to elevated temperatures (>50° C.) and lose all surface-active behavior.